Disk drive computer mass storage devices comprise a randomly accessible rotating storage medium, or disk, on which information is encoded or written by various means. In magnetic disk drives, the information is encoded as characters or dibits formed by magnetic field reversals or transitions recorded on the magnetically-hard surface of the rotating disk. The dibits of information are arranged in concentric rings called tracks. A transducer or read write "head" is positioned over the tracks to read the dibits from the tracks or to write new dibits to the tracks.
The read write head is mounted on an actuator arm which is attached to a voice coil motor ("VCM"). The VCM moves the head and actuator arm across the disk surface at very high speeds to perform seek operations in which the head is positioned over a selected track. The VCM also maintains the head over a selected track while reading or writing information. A servo system controller is the general term used to describe that portion of the disk drive which is responsible for achieving all of the functionality necessary for reading and writing information in response to requests from a computer to which the disk drive is connected for information storage purposes.
Along each track, the dibits are arranged linearly about a center line of the track. The dibit information is usually organized into sectors or fields of predetermined length along each track. A field of data information is usually preceded by a field of control information that may be used to verify the position of the head before read or write operations. The data information fields may also include an error correction code which aids in correcting errors that may occur when information is read.
In embedded servo disk drives position verification and control information is contained in a servo field which is recorded in the tracks at the time of manufacture of the disk by using a high precision servo writer or by other well known techniques. The servo field information is used to perform continuous on-track positioning of the head with respect to a center line of the track by reading and responding to the control information contained within the servo fields. The servo fields are interspersed with data fields in which data information is recorded.
The control information imbedded in the embedded servo field typically includes a preamble which delimits the beginning of a servo field, a servo address mark ("SAM") which indicates that a valid servo field has been detected, a servo synch mark ("SSM") which is used to establish and maintain synchronization over reading and writing operations, an index mark which indicates a single reference point common to all the tracks or a band of tracks on the disk, and a track number code which is a Gray coded integer value of the track currently spanned by the read write head.
The embedded servo field also typically includes off-track burst information which is written on the track when the disk drive is manufactured. The off-track bursts, which comprise dibits, are physically positioned at precise intervals and locations with respect to the track center line to provide the servo system controller with information relative to the fractional track to track displacement of the head with respect to the track center line. This positional information is generated by the head reading the off-track bursts and supplying signals having magnitudes related to the bursts read. Typically there are four off-track bursts, and the information obtained by reading the bursts is sometimes referred to as quadrature signals, quadrature information or quadrature data. In the typical disk drive the quadrature data is utilized by a data processor associated with the servo system controller to generate, calculate and provide control signals to the VCM to position the head over the track center line.
Disk drives which do not embed the servo field control information in the tracks along with the data fields of data information, usually have multiple disks or disk recording surfaces. A head moves over each recording surface, and all of the heads are ganged together for movement in unison. One of the recording surfaces contains only the servo control information which is read to control the position of ganged heads over all of the tracks. The servo control information may include off-track bursts.
It can be very difficult to maintain the head precisely located on the track center line, due to such adverse influences as rotary perturbations, run-out, vibrations, movements of the disk drive forming a part of a computer subsystem (a significant effect with portable computers) or the like. If the head is not maintained at a position located on the center line of the track, it is more likely that reading and writing errors could occur. It is therefore one of the primary functions of the servo system controller to position and maintain the heads over the track center line to obtain the best performance under reading and writing operations.
The term transport delay generally refers to the amount of time which is required by the servo system controller to detect a significant off-track condition and correct the error. Because of the inertia and other finite delay aspects of moving the mechanical elements of head, actuator arm and the VCM, as well as the time delay inherent in control feedback loops to recognize the error, calculate the magnitude of the error and establish control signals adequate to correct for the error, and other factors, transport delay is an important qualification regarding the performance of a disk drive. Due to the trend toward increased disk rotational speeds and increased dibit and track-to-track density to reduce seek times and latency, transport delay can have a significant impact on data throughput or "bandwidth" performance of the disk drive. Longer transport delays result in lower bandwidth of the disk drive, because more time is required to detect and calculate corrections to effect the position of the head and actuator.